Negative-electrode material for secondary battery, and secondary battery

Abstract

This negative-electrode material for a secondary battery comprises silicon-containing particles and a coating layer that covers at least a part of the surface of the silicon-containing particles. The silicon-containing particles include an ion-conductive phase and a silicon phase dispersed in the ion-conductive phase. The coating layer includes a lithium sulfonate compound and a hydrophobic polymer compound.

Description

Negative electrode material for secondary battery and secondary battery

[0001] The present disclosure relates to a negative electrode material for a secondary battery and a secondary battery.

[0002] Non-aqueous electrolyte secondary batteries, typified by lithium ion secondary batteries, are used as power sources for electronic devices such as mobile terminals, and as power sources for vehicles such as electric vehicles. Graphite is generally used as the negative electrode active material for non-aqueous electrolyte secondary batteries.

[0003] In recent years, the use of negative electrode active materials with a higher capacity density (capacity per unit mass) than graphite has been investigated for non-aqueous electrolyte secondary batteries, such as composite particles comprising a silicate phase and a silicon phase dispersed within the silicate phase.

[0004] In Patent Document 1, Li, Si, and M ｘ (M ｘ The present invention relates to a silicate phase containing a silicate phase containing at least one element selected from the group consisting of an alkali metal, an alkaline earth metal, and an element other than Si, silicon particles dispersed in the silicate phase, and metal particles dispersed in the silicate phase, the metal particles being composed mainly of at least one metal or alloy selected from the group consisting of Fe, Cr, Ni, Mn, Cu, Mo, Zn, and Al, and the content of each element in the silicate phase relative to the total of elements other than oxygen is 3 to 45 mol % for Li, 40 to 78 mol % for Si, and ｘ A negative electrode active material for a secondary battery has been proposed in which the content of Zn is 1 to 40 mol %.

[0005] In Patent Document 2, Li, Si, and M ｘ (M ｘ and oxide particles dispersed in the silicate phase, the oxide particles including at least one of oxide particles containing Zr, oxide particles containing Ce, oxide particles containing Ca, oxide particles containing Al, oxide particles containing Fe, oxide particles containing Mg, oxide particles containing Ti, and oxide particles containing W, the content of each element relative to the total of elements other than oxygen in the silicate phase being 3 to 45 mol % for Li, 40 to 78 mol % for Si, and ｘA negative electrode active material for a secondary battery has been proposed in which the content of Zn is 1 to 40 mol %.

[0006] International Publication No. 2019 / 151016 Pamphlet International Publication No. 2019 / 151026 Pamphlet

[0007] The silicate phase of the composite particles tends to be gradually eroded due to side reactions in batteries containing non-aqueous electrolytes, which causes deterioration of the composite particles and reduces the cycle characteristics of the battery.

[0008] The negative electrode active materials described in Patent Documents 1 and 2 can suppress to some extent the destruction of particles that accompanies the expansion and contraction of silicon during charge and discharge, but the suppression of side reactions is still insufficient.

[0009] In view of the above, one aspect of the present disclosure relates to a negative electrode material for a secondary battery, comprising: silicon-containing particles; and a coating layer covering at least a portion of the surface of the silicon-containing particles, wherein the silicon-containing particles include an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase; and the coating layer includes a lithium sulfonate compound and a hydrophobic polymer compound.

[0010] Another aspect of the present disclosure relates to a negative electrode material for a secondary battery, comprising: silicon-containing particles; and a coating layer covering at least a portion of a surface of the silicon-containing particles, wherein the silicon-containing particles include an ion-conducting phase and a silicon phase dispersed in the ion-conducting phase; and the coating layer includes a lithium sulfonate compound and a water-insoluble polymer compound.

[0011] Yet another aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode contains the above-described negative electrode material for secondary batteries.

[0012] According to the present disclosure, it is possible to suppress deterioration in the cycle characteristics of a secondary battery having a negative electrode containing silicon-containing particles.

[0013] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0014] 1 is a cross-sectional view schematically illustrating a negative electrode material for a secondary battery according to an embodiment of the present disclosure; 2 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away;

[0015] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more materials may be used in combination.

[0016] An anode material for a secondary battery according to one embodiment of the present disclosure comprises silicon-containing particles and a coating layer covering at least a portion of the surface of the silicon-containing particles. The silicon-containing particles include an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase. The coating layer includes a lithium sulfonate compound and a hydrophobic polymer compound. The coating layer is a mixed layer of the lithium sulfonate compound and the hydrophobic polymer compound. Hereinafter, the silicon-containing particles are also referred to as "composite particles."

[0017] The ion-conducting phase may include, for example, at least one selected from the group consisting of a silicate phase, a silicon oxide phase, and a carbon phase. Hereinafter, at least one of the silicate phase and the silicon oxide phase will also be referred to as a "silicon compound phase." Composite particles in which a silicon phase is dispersed within a silicate phase will also be referred to as a "silicate phase-containing composite particle." Composite particles in which a silicon phase is dispersed within a silicon oxide phase will also be referred to as a "silicon oxide phase-containing composite particle." Composite particles in which a silicon phase is dispersed within a carbon phase will also be referred to as a "carbon phase-containing composite particle."

[0018] By coating the surfaces of the composite particles (ion-conducting phase) with the lithium sulfonate compound, the composite particles are protected from the non-aqueous electrolyte, side reactions with the non-aqueous electrolyte are suppressed, and the accompanying erosion of the ion-conducting phase is suppressed, thereby suppressing deterioration of the composite particles due to the erosion and thereby reducing the deterioration of the cycle characteristics of the secondary battery.

[0019] By mixing the lithium sulfonate compound with the hydrophobic polymer compound in the coating layer, the retention of the lithium sulfonate compound on the surface of the composite particles is improved, and the surface of the composite particles is effectively coated with the lithium sulfonate compound, thereby achieving a significant effect of inhibiting the erosion of the ion-conducting phase by the lithium sulfonate compound.

[0020] The negative electrode is fabricated, for example, by preparing a slurry containing the negative electrode material and water, applying the slurry to a negative electrode current collector, drying it, and optionally rolling it to form a negative electrode mixture layer. The presence of a hydrophobic polymer compound on the surface of the negative electrode material suppresses the dissolution of the lithium sulfonate compound in water during slurry preparation. The hydrophobic polymer compound is preferably a material that combines excellent binding properties with hydrophobicity.

[0021] If a slurry is prepared by adding a lithium sulfonate compound, a hydrophobic polymer compound, and water to composite particles, the lithium sulfonate compound will dissolve in water, making it difficult to efficiently support the lithium sulfonate compound on the surfaces of the composite particles.

[0022] (Lithium Sulfonate Compound) The lithium sulfonate compound is a lithium salt of a sulfonic acid compound. The sulfonic acid compound has a sulfonic acid group (SO ３ The sulfonic acid compound may be a monosulfonic acid compound or a disulfonic acid compound.

[0023] The lithium sulfonate compound is preferably a compound represented by the following general formula (1): In general formula (1), R is an n-valent aliphatic hydrocarbon group having 1 to 5 carbon atoms, and n is 1 or 2.

[0024]

[0025] The lithium sulfonate compound more preferably contains at least one selected from the group consisting of lithium methanesulfonate, lithium ethanesulfonate, and lithium propanesulfonate, and among these, it is particularly preferable to contain lithium methanesulfonate.

[0026] The amount of the lithium sulfonate compound covering the surface of the composite particles (hereinafter also referred to as the supported amount of the lithium sulfonate compound) may be 1 part by mass or more, 1 part by mass or more and 10 parts by mass or less, 1 part by mass or more and 6 parts by mass or less, or 2 parts by mass or more and 6 parts by mass or less, relative to 100 parts by mass of the composite particles.

[0027] When the amount of the lithium sulfonate compound supported is 1 part by mass or more, the surface of the composite particle can be sufficiently coated with the lithium sulfonate compound, and the effect of suppressing side reactions caused by the lithium sulfonate compound can be easily achieved. When the amount of the lithium sulfonate compound supported is 6 parts by mass or less, a low-resistance negative electrode material (coating layer) can be easily obtained. When forming a coating layer containing a lithium sulfonate compound and a hydrophobic polymer compound, the lithium sulfonate compound can be supported on the surface of the composite particle in an amount within the above range.

[0028] (Hydrophobic polymer compound) The hydrophobic polymer compound preferably has good binding properties and thermal melting properties. In this case, the hydrophobic polymer compound can firmly support the lithium sulfonate compound on the surface of the composite particles, and the effect of suppressing side reactions caused by the lithium sulfonate compound can be stably obtained. The hydrophobic polymer compound is almost insoluble in water.

[0029] From the viewpoint of stability against non-aqueous electrolytes, the hydrophobic polymer compound preferably contains a fluororesin. The fluororesin preferably contains at least one selected from the group consisting of polyvinylidene fluoride (PVDF), polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer, chlorotrifluoroethylene-ethylene copolymer, and polychlorotrifluoroethylene. Of these, PVDF is more preferred. PVDF has good binding properties, a low melting point, and can form a coating layer at a low heat treatment temperature.

[0030] The hydrophobic polymer compound may include, in addition to polyvinylidene fluoride, a polymer containing vinylidene fluoride units. Examples of polymers containing vinylidene fluoride units include copolymers of vinylidene fluoride with other monomers. Examples of other monomers include hexafluoropropylene (HFP) and tetrafluoroethylene (TFE). Polymers containing vinylidene fluoride units include polyvinylidene fluoride and its modified products, vinylidene fluoride-hexafluoropropylene copolymers, and vinylidene fluoride-chlorotrifluoroethylene copolymers. In polymers containing vinylidene fluoride units, the content of vinylidene fluoride units is, for example, 30 mol% or more, and may be 50 mol% or more.

[0031] The amount of hydrophobic polymer compound covering the surface of the composite particle (hereinafter also referred to as the supported amount of hydrophobic polymer compound) may be 1 part by mass or more, 1 part by mass or more and 10 parts by mass or less, 1 part by mass or more and 6 parts by mass or less, or 2 parts by mass or more and 6 parts by mass or less, relative to 100 parts by mass of the composite particle.

[0032] When the amount of the hydrophobic polymer compound supported is 1 part by mass or more, the effect of the hydrophobic polymer compound in improving the retention of the lithium sulfonate compound on the surface of the composite particles is sufficiently obtained.When the amount of the hydrophobic polymer compound supported is 6 parts by mass or less, a low-resistance negative electrode material (coating layer) is easily obtained.

[0033] When the coating layer contains a lithium sulfonate compound and a fluororesin (for example, PVDF), the amounts of the lithium sulfonate compound and the fluororesin supported (amounts relative to 100 parts by mass of composite particles) can be determined by the following method.

[0034] The negative electrode material is washed with N-methyl-2-pyrrolidone (NMP) to dissolve the fluororesin, and the difference in mass of the NMP before and after dissolution is determined as the mass of the fluororesin. The remainder that is not dissolved in NMP is then washed with water to dissolve the lithium sulfonate compound. The mass of the lithium sulfonate compound dissolved in water is determined by quantitative analysis such as ICP emission spectroscopy.

[0035] When the negative electrode material comprises silicon compound phase-containing composite particles, the remaining mass that is not dissolved in water and NMP is determined as the mass of the silicon compound phase-containing composite particles. When the negative electrode material comprises silicon compound phase-containing composite particles having a conductive layer, the remaining mass that is not dissolved in water and NMP is subjected to quantitative carbon analysis using a carbon-sulfur analyzer or the like. The determined carbon amount is derived from the carbon material of the conductive layer. The value obtained by subtracting the mass of carbon determined by analysis from the remaining mass that is not dissolved in water and NMP is determined as the mass of the silicon compound phase-containing composite particles.

[0036] Using the mass of the fluororesin and the mass of the composite particles determined above, the amount of fluororesin supported is calculated by (mass of fluororesin / mass of composite particles) x 100. Using the mass of the lithium sulfonate compound and the mass of the composite particles determined above, the amount of lithium sulfonate compound supported is calculated by (mass of lithium sulfonate compound / mass of composite particles) x 100.

[0037] The thickness of the coating layer is preferably thin enough that it does not substantially affect the average particle size of the composite particles. From the viewpoint of protecting the composite particles from the electrolyte, the thickness of the coating layer is preferably 1 nm or more. From the viewpoint of suppressing an increase in resistance, the thickness of the coating layer is preferably 300 nm or less. The thickness of the coating layer may be smaller than the thickness of the conductive layer described below. The thickness of the coating layer can be measured by observing the cross section of the composite particles using an electron microscope. The electron microscope used is a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0038] (Conductive Layer) From the viewpoint of improving conductivity, a conductive layer containing a conductive carbon material may be interposed between the composite particle and the coating layer. That is, the coating layer may be formed so as to cover the conductive layer on the surface of the composite particle. The thickness of the conductive layer is preferably thin enough not to substantially affect the average particle size of the composite particle. From the viewpoint of ensuring conductivity, the thickness of the conductive layer is preferably 1 nm or more. From the viewpoint of suppressing an increase in resistance, the total thickness of the coating layer and the conductive layer is preferably 300 nm or less. The thickness of the conductive layer can be measured in the same manner as in the case of the coating layer.

[0039] The conductive layer is formed by mixing a raw material of a conductive carbon material with composite particles and firing the mixture to carbonize the raw material of the conductive carbon material. Examples of the raw material of the conductive material include coal pitch, coal tar pitch, petroleum pitch, and phenolic resin. The mixture of the raw material of the conductive carbon material and the composite particles is fired, for example, in an inert atmosphere (e.g., an argon or nitrogen atmosphere). The firing temperature is preferably 450°C or higher and 1000°C or lower. Within this temperature range, a highly conductive layer is easily formed in a silicate phase with low crystallinity. The firing temperature is preferably 550°C or higher and 900°C or lower, more preferably 650°C or higher and 850°C or lower. The firing time is, for example, 1 hour or higher and 10 hours or lower.

[0040] (Method for Producing a Coating Layer) The method for producing a coating layer includes, for example, a mixing step of dry-mixing composite particles (or composite particles having a conductive layer) with powders of a lithium sulfonate compound and a hydrophobic polymer compound to obtain a mixture, and then a heat treatment step of heat-treating the mixture. When the composite particles have a conductive layer on their surfaces, the coating layer is formed on the surfaces of the composite particles via the conductive layer.

[0041] The mixing step produces an intermediate (mixture) in which a mixture of a lithium sulfonate compound and a hydrophobic polymer compound adheres to the surface of the composite particles. A ball mill method can be used for the dry mixing step. The amount of the lithium sulfonate compound added may be 1 part by mass or more and 6 parts by mass or less per 100 parts by mass of the composite particles. The amount of the hydrophobic polymer compound added may be 1 part by mass or more and 6 parts by mass or less per 100 parts by mass of the composite particles.

[0042] In the heat treatment step, the intermediate is heated, for example, to a temperature equal to or higher than the melting point of the hydrophobic polymer compound. The heat treatment step liquefies the hydrophobic polymer compound in the mixture, and the hydrophobic polymer compound penetrates and diffuses around the composite particles and the lithium sulfonate compound particles, filling the gaps between the composite particles and the lithium sulfonate compound particles, as well as the gaps between the lithium sulfonate compound particles themselves. This enhances the retention of the lithium sulfonate compound on the surface of the composite particles. In this way, a coating layer, which is a mixed layer of the lithium sulfonate compound and the hydrophobic polymer compound, is formed. The mixture after the heat treatment is crushed to obtain composite particles with a coating layer.

[0043] The heat treatment is desirably carried out at a temperature equal to or higher than the melting point of the hydrophobic polymer compound and equal to or lower than the decomposition temperature of the hydrophobic polymer compound. When polyvinylidene fluoride (PVDF) is used as the hydrophobic polymer compound, the heat treatment temperature may be equal to or higher than the melting point (150°C to 170°C) of PVDF and equal to or lower than the decomposition temperature (340°C), for example, 200°C to 250°C is preferable. The heat treatment may be carried out in an inert gas atmosphere. The heat treatment time may be, for example, about 1 to 3 hours.

[0044] The particle sizes of the lithium sulfonate compound and the hydrophobic polymer compound added in the mixing step are preferably smaller than the particle size of the composite particles. In this case, the surfaces of the composite particles (coating layers) are easily covered uniformly with the lithium sulfonate compound and the hydrophobic polymer compound. The average particle sizes of the lithium sulfonate compound and the hydrophobic polymer compound may be 1 to 100 μm or 1 to 10 μm, respectively.

[0045] When the negative electrode material is subjected to surface analysis by X-ray photoelectron spectroscopy (XPS), a peak derived from the lithium sulfonate compound is observed in the surface portion (including the outermost surface) of the coating layer. Specifically, this peak has a binding energy of approximately 165 to 170 eV and an intensity (c / s) of 200 to 1000.

[0046] According to another embodiment of the present disclosure, a negative electrode material for a secondary battery includes silicon-containing particles and a coating layer covering at least a portion of the surface of the silicon-containing particles. The silicon-containing particles include an ion-conducting phase and a silicon phase dispersed within the ion-conducting phase. The coating layer includes a lithium sulfonate compound and a water-insoluble polymer compound. The coating layer is a mixed layer of the lithium sulfonate compound and the water-insoluble polymer compound.

[0047] The coating layer containing a lithium sulfonate compound and a water-insoluble polymer compound also provides the same effects as the coating layer containing a lithium sulfonate compound and a hydrophobic polymer compound. By covering the surface of the composite particle with a coating layer containing a lithium sulfonate compound and a water-insoluble polymer compound, side reactions are suppressed, and deterioration of cycle characteristics due to the side reactions is suppressed.

[0048] (Water-insoluble polymer compound) When 1 g of the water-insoluble polymer compound is added to 100 g of water at 25° C. and the water is thoroughly stirred, the amount of the polymer compound that dissolves is less than 0.02 g (or 0.01 g or less). Examples of the water-insoluble polymer compound include hydrophobic polymer compounds (e.g., fluororesins), as well as polymethyl methacrylate, polyethylene terephthalate, polybutylene terephthalate, polyacrylonitrile, polyimide, polyamide, polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polycarbonate.

[0049] (Composite Particles) The composite particles have a structure in which a silicon phase is dispersed within an ion-conducting phase (matrix). The ion-conducting phase alleviates the stress associated with the expansion and contraction of the silicon phase during charge and discharge, suppressing cracking and fracture of the composite particles. This allows for both high capacity due to the inclusion of silicon and improved cycle characteristics. The ion-conducting phase may include a silicon compound phase (at least one of a silicate phase and a silicon oxide phase) or a carbon phase. The ion-conducting phase may be composed of one phase or multiple phases.

[0050] The silicon oxide phase is composed of a compound of Si and O. The main component (e.g., 95 to 100% by mass) of the silicon oxide phase may be silicon dioxide. The silicate phase is composed of a compound containing a metal element, silicon (Si), and oxygen (O). The silicate phase preferably contains at least lithium silicate. In this case, lithium ions can easily enter and exit the silicate phase. Among the silicon oxide phase and the silicate phase, those containing the silicate phase as the main component are preferred because of their small irreversible capacity. Here, the "main component" refers to a component that accounts for 50% by mass or more of the total mass of the silicon compound phase, and may account for 70% by mass or more. The ion-conducting phase may be composed of a silicon compound phase, containing a lithium silicate phase as the main component, and a small amount of a silicon oxide phase.

[0051] The composite particles may be composite particles containing a silicate phase and a silicon phase dispersed within the silicate phase (silicate phase-containing composite particles). The silicate phase-containing composite particles can be obtained, for example, by pulverizing a mixture of silicate and raw material silicon while stirring it in a ball mill or the like to form fine particles, then heat-treating the mixture in an inert atmosphere and pulverizing the sintered product obtained by the heat treatment.

[0052] The silicate phase preferably contains at least one of an alkali metal element (a Group 1 element other than hydrogen in the long periodic table) and a Group 2 element in the long periodic table. Alkali metal elements include lithium (Li), potassium (K), sodium (Na), etc. Group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), etc. The silicate phase may further contain other elements such as rare earth elements such as lanthanum (La), aluminum (Al), and boron (B).

[0053] Lithium silicate is a silicate containing lithium (Li), silicon (Si), and oxygen (O). The atomic ratio of O to Si in lithium silicate: O / Si is, for example, greater than 2 and less than 4. When the O / Si ratio is greater than 2 and less than 4 (z in the formula described below is 0<z<2), this is advantageous in terms of the stability of the silicate phase and lithium ion conductivity. Preferably, the O / Si ratio is greater than 2 and less than 3. The atomic ratio of Li to Si in lithium silicate: Li / Si is, for example, greater than 0 and less than 4.

[0054] The composition of lithium silicate is represented by the formula: Li ２ｚ SiO ２＋ｚ (0<z<2). From the viewpoints of stability, ease of preparation, lithium ion conductivity, etc., z preferably satisfies the relationship 0<z<1, and more preferably z=1 / 2. Lithium silicate satisfying z=1 / 2 is Li ２ Si ２ O ５ It can be expressed as:

[0055] The average particle size of the fine silicon phase (before the first charge) dispersed within the silicate phase may be 500 nm or less, 200 nm or more, or 50 nm or less. By appropriately miniaturizing the silicon phase in this way, volume change during charge and discharge is reduced, improving structural stability. Furthermore, uniform expansion and contraction of the silicon phase is achieved, suppressing particle cracking and improving cycle performance. The average particle size of the silicon phase is measured by observing the cross section of the negative electrode material using a SEM or TEM. Specifically, it is determined by averaging the maximum diameters of 100 randomly selected silicon phases.

[0056] The silicon phase dispersed within the silicate phase is a particulate phase of simple silicon (Si) and is composed of a single or multiple crystallites. The crystallite size of the silicon phase may be 5 nm or more and 50 nm or less. When the crystallite size of the silicon phase is 50 nm or less, the amount of volume change due to expansion and contraction of silicon particles during charging and discharging can be reduced, further improving cycle characteristics. The crystallite size of the silicon phase is calculated using the Scherrer equation from the half-width of the diffraction peak assigned to the Si(111) plane in the X-ray diffraction (XRD) pattern.

[0057] From the viewpoint of increasing capacity and improving cycle characteristics, the content of the silicon phase in the silicate phase-containing composite particles may be 30% by mass or more and 90% by mass or less, or 35% by mass or more and 75% by mass or less.

[0058] The composition of the silicate phase-containing composite particles can be determined, for example, by the following analytical method.

[0059] <EDX> From the cross-sectional image of the backscattered electron image of the negative electrode mixture layer, ten composite particles with a maximum particle diameter of 5 μm or more are randomly selected, and elemental mapping analysis is performed on each particle using energy dispersive X-ray (EDX). The area percentage of the target element is calculated using image analysis software. The observation magnification is preferably 2000 to 20,000 times. The measured values ​​of the area percentage of a specific element contained in the ten particles are averaged. The content of the target element is calculated from the obtained average value.

[0060] Desirable measurement conditions for cross-sectional SEM-EDX analysis are shown below. <SEM-EDX measurement conditions> Processing equipment: SM-09010 (Cross Section Polisher) manufactured by JEOL Processing conditions: Acceleration voltage 6 kV Current value: 140 μA Vacuum degree: 1×10 -3 ~2 x 10 -3Pa Measuring device: Electron microscope HITACHI SU-70 Acceleration voltage during analysis: 10 kV Field: Free mode Probe current mode: Medium Probe current range: High Anode Ap.: 3 OBJ Ap.: 2 Analysis area: 1 μm square Analysis software: EDAX Genesis CPS: 20500 Lsec: 50 Time constant: 3.2

[0061] <AES> From the cross-sectional image of the backscattered electron image of the negative electrode mixture layer, ten composite particles having a maximum particle diameter of 5 μm or more are randomly selected, and each is subjected to qualitative and quantitative elemental analysis using an Auger electron spectroscopy (AES) analyzer (e.g., JAMP-9510F manufactured by JEOL Ltd.). Measurement conditions may be, for example, an acceleration voltage of 10 kV, a beam current of 10 nA, and an analysis area of ​​20 μmφ. The content of a predetermined element contained in the ten particles is averaged to calculate the content.

[0062] During the charge / discharge process, a coating is formed on the surface of the composite particle due to decomposition of the non-aqueous electrolyte, etc. Furthermore, as described below, the composite particle may further include a conductive layer covering the surface of the composite particle. Therefore, mapping analysis using EDX or AES is performed within a range 1 μm inside from the peripheral edge of the cross section of the composite particle so that the thin coating or conductive layer is not included in the measurement range. Mapping analysis also makes it possible to confirm the distribution of the carbon material inside the composite particle. Since it becomes difficult to distinguish from decomposition products of the non-aqueous electrolyte at the end of the cycle, it is preferable to measure a sample before or at the beginning of the cycle.

[0063] <ICP> A sample of composite particles is completely dissolved in a heated acid solution (a mixed acid of hydrofluoric acid, nitric acid, and sulfuric acid), and the carbon remaining in the solution is filtered off. The filtrate is then analyzed by inductively coupled plasma emission spectroscopy (ICP) to measure the spectral intensity of each element. A calibration curve is then created using commercially available standard solutions of the elements, and the content of each element contained in the composite particles is calculated.

[0064] The contents of B, Na, K and Al contained in the silicate phase can be quantitatively analyzed in accordance with JIS R3105 (1995) (method of analysis of borosilicate glass).

[0065] In silicate phase-containing composite particles, a silicate phase and a silicon phase exist, and these can be distinguished and quantified using Si-NMR. The Si content obtained by the above method is the sum of the amount of Si constituting the silicon phase and the amount of Si in the silicate phase. The amount of Si element contained in the composite particles is distributed between the silicate phase and the silicon phase using the results of quantitative analysis by Si-NMR. Note that the standard substance required for quantification can be a mixture containing a silicate phase and a silicon phase in a predetermined ratio with known Si contents.

[0066] Desirable Si-NMR measurement conditions are shown below. <Si-NMR measurement conditions> Measurement device: Varian solid-state nuclear magnetic resonance spectrometer (INOVA-400) Probe: Varian 7 mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1H decoupled) Repetition time: 1200 sec to 3000 sec Observation width: 100 kHz Observation center: near -100 ppm Signal acquisition time: 0.05 sec Number of accumulations: 560 Sample amount: 207.6 mg

[0067] An example of a method for producing silicate phase-containing composite particles will be described in detail below.

[0068] Step (i) (Step of Obtaining Lithium Silicate) A raw material mixture containing a Si-containing raw material and a Li raw material in a predetermined ratio is used as the raw material for lithium silicate. If necessary, another element M may be added to the raw material mixture. A mixture of predetermined amounts of the raw materials is melted, and the molten liquid is passed through a metal roll to form flakes, producing lithium silicate. The flaked silicate is then crystallized by heat treatment in an air atmosphere at a temperature above the glass transition point and below the melting point. The flaked silicate can also be used without crystallization. Alternatively, a predetermined amount of the mixture can be fired at a temperature below the melting point without being melted, to produce silicate by a solid-phase reaction.

[0069] Silicon oxide can be used as the Si raw material. For example, lithium carbonate, lithium oxide, lithium hydroxide, lithium hydride, etc. can be used as the Li raw material. These may be used alone or in combination of two or more. Examples of raw materials for element M include oxides, hydroxides, carbonate compounds, hydrides, nitrates, sulfates, etc. of each element. Si raw material that has not reacted with the Li raw material may remain in the lithium silicate. The remaining Si raw material is dispersed in the lithium silicate as fine crystals of silicon oxide.

[0070] Step (ii) (Step of Obtaining Silicate Composite Particles) Next, lithium silicate is compounded with raw silicon to form composite particles. For example, the composite particles are produced through the following steps (a) to (c).

[0071] Step (a) First, raw silicon powder and lithium silicate powder are mixed in a mass ratio of, for example, 20:80 to 95:5. The raw silicon powder may be coarse silicon particles having an average particle size of several μm to several tens of μm.

[0072] Step (b): Next, using a grinding device such as a ball mill, the mixture of raw silicon and lithium silicate is ground and compounded while being finely divided. At this time, an organic solvent may be added to the mixture and wet-ground. A predetermined amount of organic solvent may be added to the grinding vessel all at once at the beginning of grinding, or a predetermined amount of organic solvent may be added to the grinding vessel intermittently in multiple batches during the grinding process. The organic solvent serves to prevent the material to be ground from adhering to the inner wall of the grinding vessel.

[0073] As the organic solvent, alcohols, ethers, fatty acids, alkanes, cycloalkanes, silicate esters, metal alkoxides, etc. can be used.

[0074] The raw silicon may be coarse silicon particles having an average particle size of several μm to several tens of μm. The silicon particles finally obtained are preferably controlled so that the crystallite size calculated from the half-width of the diffraction peak attributable to the Si(111) plane in the X-ray diffraction pattern using the Scherrer equation is 5 nm or more and 50 nm or less.

[0075] The raw silicon and lithium silicate may be separately microparticulated and then mixed. Alternatively, silicon nanoparticles and amorphous lithium silicate nanoparticles may be prepared without using a pulverizer and then mixed. The nanoparticles may be prepared by known methods such as a gas phase method (e.g., a plasma method) or a liquid phase method (e.g., a liquid phase reduction method).

[0076] Step (c) Next, the pulverized material is fired under pressure using a hot press or the like to obtain a sintered body. The firing is carried out, for example, in an inert atmosphere (e.g., an argon, nitrogen, or other atmosphere). The firing temperature is preferably 450°C or higher and 1000°C or lower. Within this temperature range, fine silicon particles are easily dispersed within the silicate phase with low crystallinity. During sintering, the lithium silicate softens and flows to fill the gaps between the silicon particles. As a result, a dense block-shaped sintered body can be obtained, with the silicate phase forming a sea portion and the silicon particles forming islands. The firing temperature is preferably 550°C or higher and 900°C or lower, more preferably 650°C or higher and 850°C or lower. The firing time is, for example, 1 hour or higher and 10 hours or lower.

[0077] The resulting sintered body is pulverized to obtain silicate composite particles. By appropriately selecting the pulverization conditions, silicate composite particles with a predetermined average particle size can be obtained. Steps (i) and (ii) produce composite particles having a silicate phase as a matrix and a silicon phase dispersed in the matrix.

[0078] The composite particles may be composite particles containing a silicon oxide phase and a silicon phase dispersed within the silicon oxide phase (silicon oxide phase-containing composite particles). The silicon oxide phase-containing composite particles are represented, for example, by the formula SiOX (0.5≦X<1.6). The silicon oxide phase-containing composite particles can be obtained, for example, by heat-treating silicon monoxide and separating it into a silicon oxide phase and a fine silicon phase by a disproportionation reaction.

[0079] The composite particles may be composite particles containing a carbon phase and a silicon phase dispersed in the carbon phase (carbon phase-containing composite particles). The carbon phase-containing composite particles can be obtained, for example, by pulverizing a mixture of a carbon source and raw silicon while stirring it in a ball mill or the like to form fine particles, then heat-treating the mixture in an inert atmosphere and pulverizing the sintered product obtained by the heat treatment. Examples of the carbon source that can be used include sugars such as carboxymethyl cellulose (CMC) and water-soluble resins such as polyvinylpyrrolidone.

[0080] Because the carbon phase is electrically conductive, in a carbon phase-containing composite particle, even if voids are formed around the composite particle, contact between the composite particle and its surroundings is likely to be maintained. As a result, capacity loss due to repeated charge / discharge cycles is likely to be suppressed. The carbon phase may be composed of amorphous carbon (i.e., amorphous carbon). The amorphous carbon may be hard carbon, soft carbon, or other. Amorphous carbon generally refers to a carbon material in which the average interplanar spacing d002 of the (002) plane measured by X-ray diffraction exceeds 0.34 nm.

[0081] From the viewpoint of increasing capacity and improving cycle characteristics, the content of the silicon phase in the carbon phase-containing composite particles may be 30% by mass or more and 80% by mass or less, or 40% by mass or more and 70% by mass or less.

[0082] The average particle size of the composite particles is, for example, 1 μm or more and 25 μm or less, or may be 4 μm or more and 15 μm or less. Within this range, good battery performance is likely to be obtained. In this specification, the average particle size refers to the particle size (volume-average particle size) at which the volume cumulative value is 50% in the particle size distribution measured by a laser diffraction scattering method. For example, an "LA-750" manufactured by Horiba Ltd. can be used as the measuring device.

[0083] FIG. 1 shows a schematic cross section of a negative electrode material 20. The negative electrode material 20 includes a composite particle 23 (mother particle) and a coating layer 26 covering the surface of the composite particle 23. The composite particle 23 includes an ion-conducting phase 21 and a silicon phase (silicon particles) 22 dispersed within the ion-conducting phase 21. The composite particle 23 has a sea-island structure in which fine silicon phases 22 are dispersed within the matrix of the ion-conducting phase 21. The coating layer 26 contains a lithium sulfonate compound and a hydrophobic polymer compound. The negative electrode material may include other elements in addition to the composite particle and the coating layer. For example, a conductive layer may be interposed between the composite particle and the coating layer.

[0084] A secondary battery according to an embodiment of the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode contains the above-described negative electrode material for a secondary battery. The negative electrode of the secondary battery and other components will be described below.

[0085] [Negative Electrode] The negative electrode comprises, for example, a negative electrode current collector and a negative electrode mixture layer supported on the surface of the negative electrode current collector. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the negative electrode mixture is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying it. The dried coating may be rolled as necessary. The negative electrode mixture layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0086] The negative electrode mixture contains the above-mentioned negative electrode material as an essential component, and may contain, as optional components, a binder, a conductive agent, a thickener, etc. The silicon particles in the composite particles can occlude many lithium ions, thereby contributing to increasing the capacity of the negative electrode.

[0087] The negative electrode active material may further contain another active material that electrochemically absorbs and releases lithium ions. A preferred example of such another active material is a carbon-based active material. Because the composite particles expand and contract in volume with charge and discharge, a large proportion of the composite particles in the negative electrode active material is likely to result in poor contact between the negative electrode active material and the negative electrode current collector with charge and discharge. On the other hand, the combined use of the composite particles and a carbon-based active material makes it possible to achieve excellent cycle characteristics while imparting high capacity to the negative electrode due to the silicon phase. The proportion of the composite particles in the total of the composite particles and the carbon-based active material is preferably 0.5 to 15 mass%, more preferably 1 to 5 mass%. This makes it easier to achieve both high capacity and improved cycle characteristics.

[0088] Examples of carbon-based active materials include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). Among these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity. Graphite refers to a material having a graphite-type crystal structure, and includes, for example, natural graphite, artificial graphite, and graphitized mesophase carbon particles. One type of carbon-based active material may be used alone, or two or more types may be used in combination.

[0089] As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used. Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm, from the viewpoint of balancing the strength and weight of the negative electrode.

[0090] Examples of binders include fluororesin, polyolefin resin, polyamide resin, polyimide resin, vinyl resin, styrene-butadiene copolymer rubber (SBR), polyacrylic acid and its derivatives. These may be used alone or in combination of two or more. Examples of conductive agents include carbon black, conductive fiber, carbon fluoride, and organic conductive materials. These may be used alone or in combination of two or more. Examples of thickeners include carboxymethyl cellulose (CMC) and polyvinyl alcohol. These may be used alone or in combination of two or more.

[0091] Examples of the dispersion medium include water, alcohol, ether, N-methyl-2-pyrrolidone (NMP), and a mixed solvent thereof.

[0092] [Positive Electrode] The positive electrode comprises, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector. The positive electrode mixture layer can be formed by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying it. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector.

[0093] The positive electrode mixture contains a positive electrode active material as an essential component, and may contain a binder, a conductive agent, and the like as optional components.

[0094] As the positive electrode active material, a lithium composite metal oxide can be used. Examples of the lithium composite metal oxide include Li a CoO2, Li a NiO2, Li a MnO2, Lia Co b Ni 1-b O2, Li a Co b M 1-b O c , Li a Ni 1-b M b O c , Li a Mn2O4, Li a Mn 2-b M b Examples of the lithium-ion battery include LiMePO4, LiMePO4, and Li2MePO4F. Here, M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Me contains at least a transition element (e.g., at least one selected from the group consisting of Mn, Fe, Co, and Ni). Here, 0≦a≦1.2, 0≦b≦0.9, and 2.0≦c≦2.3. The value a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

[0095] The binder and conductive agent may be the same as those exemplified for the negative electrode. As the conductive agent, graphite such as natural graphite or artificial graphite may be used.

[0096] The shape and thickness of the positive electrode current collector can be selected from the shape and range corresponding to those of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0097] [Non-aqueous electrolyte] The non-aqueous electrolyte contains a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 to 2 mol / L. The non-aqueous electrolyte may contain known additives.

[0098] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, and cyclic carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0099] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 Lithium salts such as fluorine-containing acids (LiPF, LiBF, LiSbF, LiAsF, LiCF, SO, LiCF, CO, etc.), lithium salts of fluorine-containing acid imides (LiN(CF, SO), LiN(CF) SO (CF, SO), LiN(CF, SO) (CF, SO), LiN(CF, SO), etc.), and lithium halides (LiCl, LiBr, LiI, etc.). One type of lithium salt may be used alone, or two or more types may be used in combination.

[0100] [Separator] It is usually desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator can be made of a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The separator can be made of polyolefins such as polypropylene and polyethylene.

[0101] An example of the structure of a secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and a nonaqueous electrolyte are housed in an exterior body. Alternatively, instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0102] Hereinafter, the structure of a prismatic non-aqueous electrolyte secondary battery as an example of a secondary battery according to the present disclosure will be described with reference to Fig. 2. Fig. 2 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away.

[0103] The battery includes a bottomed prismatic battery case 4, and an electrode group 1 and a nonaqueous electrolyte (not shown) housed within the battery case 4. The electrode group 1 includes a long strip-shaped negative electrode, a long strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, positive electrode, and separator around a flat plate-shaped winding core and then removing the winding core.

[0104] One end of a negative electrode lead 3 is attached to the negative electrode current collector by welding or the like. The other end of the negative electrode lead 3 is electrically connected to a negative electrode terminal 6 provided on the sealing plate 5 via a resin insulating plate (not shown). The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. One end of a positive electrode lead 2 is attached to the positive electrode current collector by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. That is, the positive electrode lead 2 is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The insulating plate separates the electrode group 1 from the sealing plate 5 and also separates the negative electrode lead 3 from the battery case 4. The periphery of the sealing plate 5 fits into the open edge of the battery case 4, and the fitting portion is laser-welded. In this way, the opening of the battery case 4 is sealed with the sealing plate 5. A non-aqueous electrolyte injection hole provided in the sealing plate 5 is closed with a seal 8.

[0105] Examples Examples of the present disclosure will be specifically described below, but the present disclosure is not limited to the following examples.

[0106] Examples 1 to 3 (Preparation of Composite Particles) Lithium carbonate (Li ２ CO ３ ) and silicon dioxide (SiO ２ ) was mixed with Li at an atomic ratio of 1.05 to obtain a mixture. The mixture was fired at 800°C for 10 hours in an inert gas atmosphere to obtain lithium silicate (Li２ Si ２ O ５ The obtained lithium silicate was pulverized to an average particle size of 10 μm.

[0107] The crushed lithium silicate and raw silicon (3N, average particle size 10 μm) were mixed in a mass ratio of 40:60. The mixture was charged into a pot (SUS, volume: 500 mL) of a planetary ball mill (Fritsch, P-5). Next, 24 SUS balls (diameter 20 mm) were placed in the pot, the lid was closed, and the mixture was crushed at 200 rpm for 50 hours in an inert atmosphere.

[0108] The powder mixture was then removed from the inert atmosphere and fired at 600°C for 4 hours under pressure from a hot press to obtain a sintered body of the mixture. The sintered body of the mixture was then crushed and passed through a 40 µm mesh to obtain composite particles (silicate phase-containing composite particles). Then, composite particles with an average particle size of 10 µm were obtained using a sieve.

[0109] The crystallite size of the silicon phase was 15 nm. The silicon phase was particulate, and the average particle size of the silicon phase was 20 nm. The main component of the silicate phase was Li. ２ Si ２ O ５ The content of the silicon phase in the silicate phase-containing composite particles was 60 mass %.

[0110] (Formation of Conductive Layer) Then, 100 parts by mass of the composite particles were mixed with 3 parts by mass of coal tar pitch, and the mixture was fired at 800°C in an inert atmosphere to coat the surfaces of the composite particles with conductive carbon. In this way, a conductive layer was formed on the surfaces of the composite particles. The coating amount of the conductive layer was 3% by mass based on the total weight of the composite particles and the conductive layer.

[0111] (Preparation of negative electrode material: formation of coating layer on composite particle surface) (Dry mixing process) Powders of lithium methanesulfonate (MSL) and polyvinylidene fluoride (PVDF) were added to the composite particles having a conductive layer, and a dry mixing process was performed to attach MSL and PVDF to the surface of the composite particles having a conductive layer, thereby obtaining an intermediate. The amounts of MSL and PVDF added per 100 parts by mass of the composite particles were the values ​​shown in Table 1. The dry mixing process was performed by a ball mill method, and a rocking mill manufactured by Seiwa Giken Co., Ltd. was used as the device. The dry mixing process was performed using zirconia balls (diameter 3 mm) at room temperature (25°C) for 30 minutes.

[0112] Next, the obtained intermediate was heat-treated at 250° C. for 2 hours in an inert atmosphere to liquefy the PVDF attached to the surface of the composite particles, thereby enhancing the retention of MSL on the surface of the composite particles. In this way, a coating layer (a mixed layer of MSL and PVDF) was formed on the surface of the composite particles having the conductive layer, thereby obtaining a negative electrode material.

[0113] (Preparation of Negative Electrode) Graphite and composite particles having a conductive layer and a coating layer (negative electrode material) were mixed in a mass ratio of 90:10, and this was used as the negative electrode active material. 1 part by mass of carboxymethyl cellulose (CMC) and 1.5 parts by mass of styrene-butadiene rubber (SBR) were added to 97.5 parts by mass of the negative electrode active material, and a predetermined amount of water was further added to prepare a negative electrode slurry.

[0114] The negative electrode slurry was applied to both sides of a copper foil negative electrode current collector, the coating was dried, rolled, and then cut to a predetermined size to obtain a negative electrode having a negative electrode mixture layer formed on both sides of the negative electrode current collector. At this time, a negative electrode current collector exposed portion was provided in a part of the negative electrode.

[0115] (Preparation of Positive Electrode) 2.5 parts by mass of acetylene black and 2.5 parts by mass of polyvinylidene fluoride were added to 95 parts by mass of the positive electrode active material, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) was further added to prepare a positive electrode slurry. ０．８８ Co ０．０９ Al ０．０３ O ２ The lithium transition metal composite oxide represented by the following formula was used.

[0116] The positive electrode slurry was applied to both sides of an aluminum foil serving as a positive electrode current collector, the coating was dried, rolled, and then cut to a predetermined size to obtain a positive electrode having a positive electrode mixture layer formed on both sides of the positive electrode current collector. At this time, a positive electrode current collector exposed portion was provided in a part of the positive electrode.

[0117] (Preparation of non-aqueous electrolyte) Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 20:5:75 to obtain a mixed solvent, and lithium hexafluorophosphate was dissolved in the mixed solvent at a concentration of 1 mol / L to prepare a non-aqueous electrolyte.

[0118] (Secondary Battery Fabrication) An aluminum positive electrode lead was attached to the exposed portion of the positive electrode current collector of the positive electrode, and a nickel negative electrode lead was attached to the exposed portion of the negative electrode current collector of the negative electrode. The positive electrode and negative electrode were spirally wound with a polyolefin separator interposed therebetween and then pressed radially to produce a flat wound electrode assembly. This electrode assembly was housed in an exterior body made of an aluminum laminate sheet, and after injecting a nonaqueous electrolyte, the opening of the exterior body was sealed to obtain a secondary battery for evaluation. In Table 1, A1 to A3 refer to the secondary batteries of Examples 1 to 3.

[0119] Comparative Example 1 A secondary battery B1 was obtained in the same manner as the secondary battery A1 of Example 1, except that no coating layer was formed on the surface of the composite particles having a conductive layer.

[0120] Comparative Example 2 A secondary battery B2 was obtained in the same manner as the secondary battery A1 of Example 1, except that PVDF was not added in the dry mixing treatment step.

[0121] Comparative Example 3 A secondary battery B3 was obtained in the same manner as the secondary battery A1 of Example 1, except that MSL was not added in the dry mixing treatment step.

[0122] [Evaluation] The secondary batteries A1 to A3 and B1 to B3 obtained above were evaluated as follows.

[0123] (Initial Discharge Capacity, Amount of Initial Side Reaction) In an environment of 25°C, constant current charging was performed at a current of 0.3 C until the voltage reached 4.2 V, and then constant voltage charging was performed at a voltage of 4.2 V until the current reached 0.02 C. Thereafter, discharging was performed at a current of 0.5 C until the voltage reached 2.5 V. The initial charge capacity and discharge capacity at this time were determined. The charge / discharge capacity was determined as the capacity per 1 g of the composite particles.

[0124] Using the obtained charge capacity and discharge capacity, the initial charge / discharge efficiency E (%) was calculated according to the following formula.

[0125] Initial charge-discharge efficiency E = (discharge capacity / charge capacity) × 100

[0126] The amount of initial side reactions was calculated using the obtained initial charge-discharge efficiency E according to the following formula: where E0 is the initial charge-discharge efficiency of the secondary battery B1 of Comparative Example 1.

[0127] Amount of initial side reaction = {(100 - E) / (100 - E0)} x 100

[0128] (Cycle Degradation Rate) In an environment of 25° C., the battery was charged at a constant current of 0.3 C until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 C. Thereafter, the battery was discharged at a current of 0.5 C until the voltage reached 2.5 V. This charge / discharge cycle was counted as one cycle, and 100 cycles were performed.

[0129] Using the discharge capacity C1 at the first cycle and the discharge capacity C2 at the 100th cycle, the discharge capacity retention rate R (%) was calculated according to the following formula.

[0130] Discharge capacity maintenance rate R = (discharge capacity C2 / discharge capacity C1) x 100

[0131] The cycle deterioration rate was calculated using the obtained discharge capacity retention rate R according to the following formula: where R0 is the discharge capacity retention rate of the secondary battery B1 of Comparative Example 1.

[0132] Cycle deterioration rate = {(100 - R) / (100 - R0)} x 100

[0133] The evaluation results are shown in Table 1. In Table 1, the initial capacity is expressed as a relative value (index) when the initial capacity of the secondary battery B1 of Comparative Example 1 is set to 100.

[0134]

[0135] The secondary batteries A1 to A3 had a high initial capacity, a small amount of initial side reaction, and a small rate of cycle deterioration, and exhibited better cycle characteristics than the secondary batteries B1 to B3.

[0136] The cycle degradation rates of Batteries B2 and B3 were only slightly reduced compared to Battery B1. On the other hand, the cycle degradation rate of Battery A3 was significantly reduced compared to Battery B1. When the negative electrode materials used in the secondary batteries A1 to A3 were subjected to XPS analysis, peaks derived from the lithium sulfonate compound were observed in the surface layer portion (including the outermost surface) of the coating layer.

[0137] Examples 4 to 6, Comparative Examples 4 to 6 Carbon phase-containing composite particles were used as the composite particles. The carbon phase-containing composite particles were prepared as follows.

[0138] Coal pitch (MCP250, manufactured by JFE Chemical Corporation) as a carbon source and raw material silicon (3N, average particle size 10 μm) were mixed in a mass ratio of 50:50. The mixture was charged into a pot (SUS, volume: 500 mL) of a planetary ball mill (P-5, manufactured by Fritsch), 24 SUS balls (diameter 20 mm) were placed in the pot, the lid was closed, and the mixture was milled at 200 rpm for 50 hours in an inert atmosphere to obtain a composite of a silicon phase and a carbon source.

[0139] Next, the composite of the silicon phase and the carbon source was fired in an inert gas atmosphere to carbonize the carbon source, and a sintered product was obtained in which the silicon phase was dispersed in the carbon phase containing amorphous carbon. Thereafter, the sintered product was pulverized using a jet mill to obtain carbon phase-containing composite particles with an average particle size of 10 μm.

[0140] The silicon phase had a crystallite size of 15 nm. The silicon phase was particulate, with an average particle size of 20 nm. The silicon phase content in the carbon phase-containing composite particles was 60 mass %.

[0141] No conductive layer was formed on the surface of the carbon phase-containing composite particles. Secondary batteries A4 to A6 of Examples 4 to 6 were obtained in the same manner as secondary batteries A1 to A3 of Examples 1 to 3, respectively, except that carbon phase-containing composite particles were used instead of silicate phase-containing composite particles with a conductive layer in the negative electrode material preparation process. Furthermore, secondary batteries B4 to B6 of Comparative Examples 4 to 6 were prepared in the same manner as secondary batteries B1 to B3 of Comparative Examples 1 to 3, respectively, except for the above.

[0142] The secondary batteries A4 to A6 and B4 to B6 obtained above were evaluated in the same manner as above. Note that E0 in the formula for calculating the initial side reaction amount and R0 in the formula for calculating the cycle deterioration rate are the initial charge-discharge efficiency and discharge capacity retention rate of the secondary battery B4 of Comparative Example 4, respectively.

[0143] The evaluation results are shown in Table 2. In Table 2, the initial capacity is expressed as a relative value (index) when the initial capacity value of secondary battery B4 of Comparative Example 4 is set to 100.

[0144]

[0145] The secondary batteries A4 to A6 had a high initial capacity, a small amount of initial side reaction, and a small rate of cycle deterioration, and the secondary batteries A4 to A6 exhibited better cycle characteristics than the secondary batteries B4 to B6.

[0146] In Batteries B5 and B6, the reduction in the cycle degradation rate was very small compared to Battery B4. On the other hand, in Battery A6, the reduction in the amount of initial side reactions and the reduction in the cycle degradation rate were greater than in Batteries B5 and B6 compared to Battery B4. When XPS analysis was performed on the negative electrode materials used in secondary batteries A4 to A6, peaks derived from the lithium sulfonate compound were observed in the surface layer portion (including the outermost surface) of the coating layer.

[0147] The nonaqueous electrolyte secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like.

[0148] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0149] 1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug, 20: negative electrode material, 21: ion-conducting phase, 22: silicon phase, 23: composite particle, 26: coating layer

Claims

1. A silicon-containing particle and a coating layer covering at least a part of a surface of the silicon-containing particle, The silicon-containing particles include an ionically conductive phase and a silicon phase dispersed within the ionically conductive phase; The coating layer comprises a lithium sulfonate compound and a hydrophobic polymer compound.

2. 2. The negative electrode material for a secondary battery according to claim 1, wherein the ion-conducting phase includes at least one phase selected from the group consisting of a silicate phase, a silicon oxide phase, and a carbon phase.

3. The lithium sulfonate compound is represented by the general formula (1): 【Chemistry 1】 wherein in general formula (1), R is an n-valent aliphatic hydrocarbon group having 1 to 5 carbon atoms, and n is 1 or 2.

4. 4. The negative electrode material for secondary batteries according to claim 3, wherein the lithium sulfonate compound includes at least one selected from the group consisting of lithium methanesulfonate, lithium ethanesulfonate, and lithium propanesulfonate.

5. 3. The negative electrode material for secondary batteries according to claim 1, wherein the amount of the lithium sulfonate compound attached to the surfaces of the silicon-containing particles is 1 part by mass or more per 100 parts by mass of the silicon-containing particles.

6. The negative electrode material for secondary batteries according to claim 1 , wherein the hydrophobic polymer compound contains a fluororesin.

7. 7. The negative electrode material for secondary batteries according to claim 6, wherein the fluororesin comprises at least one selected from the group consisting of polyvinylidene fluoride, polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer, chlorotrifluoroethylene-ethylene copolymer, and polychlorotrifluoroethylene.

8. 3. The negative electrode material for a secondary battery according to claim 1, wherein an amount of the hydrophobic polymer compound attached to the surface of the silicon-containing particle is 1 part by mass or more per 100 parts by mass of the silicon-containing particle.

9. 3. The negative electrode material for secondary batteries according to claim 1, wherein a peak derived from the lithium sulfonate compound is observed in a surface portion of the coating layer when a surface analysis is performed by X-ray photoelectron spectroscopy.

10. 3. The negative electrode material for secondary batteries according to claim 1, wherein a conductive layer containing a conductive carbon material is interposed between the silicon-containing particles and the coating layer.

11. A silicon-containing particle and a coating layer covering at least a part of a surface of the silicon-containing particle, The silicon-containing particles include an ionically conductive phase and a silicon phase dispersed within the ionically conductive phase; The coating layer comprises a lithium sulfonate compound and a water-insoluble polymer compound.

12. A positive electrode, a negative electrode, and a non-aqueous electrolyte, A secondary battery, wherein the negative electrode comprises the negative electrode material for secondary batteries according to claim 1 , 2 , or 11 .

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